As is well known the ballast forming the roadbed of a railway track is susceptible to contamination and deterioration caused by the passage of trains over the track. In some areas the ballast must be reconditioned at least annually. There are numerous forms of apparatus which have been developed for this task. Typical apparatus which are used for this type operation include track undercutters to remove the ballast from beneath the tracks, ditcher wheels to remove ballast from areas alongside the tracks and cleaning screens to recover reuseable ballast from the ballast removed by the undercutters and ditcher wheels.
It will be appreciated that the rail lines which require the most frequent maintenance are the busiest lines, therefore the time available during which the tracks may be blocked by apparatus reconditioning the ballast is quite limited. Therefore it is imperative that the reconditioning proceed as rapidly as possible. Typical ditcher wheels may remove ballast from alongside the tracks at speeds up to 5,000 feet per hour and typical undercutters may operate at slightly reduced speeds. However when the ballast from the undercutter and ditcher wheels are fed to a cleaning screen, the rate of progress is limited by the capacity of the screen. Typical screen capacity limits the forward rate of travel in such instances to about 1,000 feet per hour. The shortcomings of such machines are well known and are fully discussed in U.S. Pat. No. 4,534,415. U.S. Pat. No. 4,534,415 purports to improve the speed of the operation by providing a further ballast screening installation, mounted on the apparatus frame, which may thus effectively double the capacity of the cleaning system. While such an apparatus seems suitable for its intended purpose, it leaves something to be desired in terms of economy and efficiency in that the apparatus is appreciably more complex than the instant invention.